FIG. 10 is a block diagram showing one example of an oscillation circuit. This oscillation circuit includes a bias voltage generating unit 201, a clock signal generating unit 202, a ramp voltage generating unit 203, resistors Rx and Ry and a capacitor Cy.
The bias voltage generating unit 201 is incorporated in a semiconductor device 200 and applies a predetermined bias voltage Vx (for example, 1.5V) to an external terminal Tx.
The clock signal generating unit 202 is incorporated in the semiconductor device 200 and generates a clock signal CK of an oscillation frequency Fosc based on a reference current Ix flowing into the external terminal Tx.
The ramp voltage generating unit 203 is incorporated in the semiconductor device 200 and generates a triangular ramp voltage Vy (for example, 1V≦Vy≦2V) by charging/discharging the capacitor Cy externally attached to the external terminal Ty.
In the oscillation circuit as configured above, if the resistor Ry is not connected between the external terminal Tx and the external terminal Ty, the reference current Ix has a constant value (=Vx/Rx) uniquely determined depending on the resistance of the resistor Rx externally attached to the external terminal Tx. Further, the oscillation frequency Fosc of the clock signal CK also has a constant value.
On the other hand, when the resistor Ry is connected between the external terminal Tx and the external terminal Ty, the reference current Ix has a variable value (=Vx/Rx+(Vx−Vy)/Ry) periodically varied depending on the ramp voltage Vy. Further, the oscillation frequency Fosc of the clock signal CK also has a variable value. Accordingly, the oscillation frequency Fosc of the clock signal CK can be spread to reduce noise intensity.
However, the oscillation circuit has many parameters (Vx, Vy, Rx and Ry) included in a calculation equation for the reference current Ix in the frequency spreading and has a possibility of deviation of the oscillation frequency Fosc of the clock signal CK due to a variation of each parameter. In addition, this oscillation circuit has a problem of an increase in set size and cost due to an increase in the number of components.